The invention relates to a process for coating substrates made of a transparent material, for example floatglass, including a transparent, dielectric layer having a low refractive index (n &lt;1.7) at a high rate of layer deposition (&gt;6.0 A cm.sup.2 /W sec) by means of constant voltage cathode sputtering in a vacuum chamber, including a cathode which is, on one of its surfaces, provided with the material (target) to be sputtered and to be deposited on the substrate.
It is known to deposit thin layers of a chemical compound of a metal with oxygen or nitrogen or other gaseous materials, in the form of oxides or nitrides etc. by means of reactive high-capacity cathode sputtering (magnetron sputtering). In this process a metal containing, especially an electrically conductive material, is used as a target and the desired compound is formed when a part or the entire sputtering atmosphere is replaced by a second gaseous component during sputtering. In the gas discharge the added, reactive gas required for the sputtering process is excited and combined with the sputtered metal of the target such that a chemical reaction between the metal and the admitted gas is effected. This causes the desired compound to be deposited on the substrate and the other surfaces in the area of the gas discharge.
An electrically conductive target is to be preferred to a non-conductive target since the gas discharge can be maintained in the form of a direct current (DC) discharge as opposed to a high-frequency discharge which is to be used for non-conductive targets. The DC-discharge has the advantage that the apparatus required are significantly less complex and, hence, less expensive and it also offers the possibility to build sputtering cathodes having a length of 4 m which permit large-scale uniform coating of very large surfaces. The basics of Hf-sputtering and the apparatus involved limit the length of the cathodes to 1 m.
The deposition of dielectric layers, which are absorption-free or near-absorption-free in the visible spectral range, by means of reactive DC-magnetron-sputtering is, however, limited since the layers to be produced often are electrically insulating and, hence, can affect the sputtering process or even cause a break down. Further, the chemical reaction for forming the dielectric layer also takes place on the target surface which then causes the desired chemical compound to be deposited on the target surface, which, in turn, can drastically change the characteristic lines of discharge of the sputtering (hysteresis) and lead to an unstable process.
The above mentioned reasons determine in practice that only a few materials in the form of oxides can be sputtered with good success by means of the reactive DC-magnetron process. This includes materials which even in the form of oxides exhibit a certain electrical conductivity such as indium oxide, tin oxide, indium-tin oxide, zinc oxide. All of the aforesaid are materials which, under favorable manufacturing conditions, exhibit significant electrical conductivity (transparent, conductive oxides). Moreover, these materials exhibit a relatively low reactivity with respect to oxygen which permits suppressing the formation of insulating layers on the cathode surface to such an extent that a continuous coating is possible.
All of the above mentioned material can be used to produce sufficiently absorption-free layers in order to achieve, by means of interference effects, simple reflection reducing effects which, are required, for example, in architecture for sun protection panes or heat insulating panes. Such layers are know from the German DE-OS 33 07 661 , to which U.S. Pat. No. 4,548,691 corresponds or the EP-OS 104 870 and are manufactured in large industrial scale. It is a disadvantage that these materials which can be easily sputtered as transparent oxides have a refractive index of approximately .gtoreq.1.9; this is a significant restriction regarding optical adjustment (filter function) of layer packages since interference effects are, on the one hand, dependent on layer thickness and, on the other hand, on the refractive index of the layer used. Hence, properties which lead to a significant improvement of the function of these layers and are theoretically and in laboratory proportions possible cannot be produced since the materials required having a low refractive index cannot be produced at all or cannot cost-efficiently be produced in industrial proportions.
Especially silicon dioxide (n.TM.1.45), aluminum oxide (n.TM.1.7) and some fluorides (MgF n.TM.1.35) are known as dielectric materials having a low refractive index. In the form of thin layers these materials--especially the oxides--can be prepared from suitable targets by means of conventional or high frequency sputtering. When used for filter or reflection reducing layers they are, thus prepared, often used for small substrate dimensions (eye glasses). It has also been suggested to sputter these materials reactively and in particular by direct voltage. Especially the oxides are of interest for the coating of large surfaces since fluorides, due to aggressiveness of the fluorine component, would require in the sputtering gas special measures against excessive corrosion of the most metallic recipients and pumps.
The above mentioned oxides include the problem that the materials in elementary form (Al, Si) exhibit an extreme reactivity regarding oxygen and, further, also form very good insulating layers. Moreover, Si poses the problem that it is practically non-conductive at room temperature and, hence, doped Si must be used as a target in order to be able to operate with DC in the first place.
In order to suppress the forming of an insulating layer on the target surface during sputtering, arrangements were suggested (DE-OS 33 31 707 and DE-OS 35 21 053) which serve to cause the chemical reaction to occur at the substrate surface. In particular, the reaction balance is shifted by disposing a geometric diaphragm and a controlled gas inlet such that suboxides are formed at the target surface which still have a sufficient residual conductivity and thus do not adversely affect the discharging process. However, a preferably transparent and stoichiometric oxide is formed at the substrate surface.
These arrangements, however, imply that a relatively large portion of the sputtered material for the formation of the layer is lost, since a significant portion of the material is deposited on the geometric diaphragms included. Furthermore, due to the diaphragm aperture as well as the more difficult process control, the latter being unstable, the rate of deposition is limited to values ranging about 1/3 of the coating rates which are achieved for the above said materials having a high refractive index. This means the devices and processes indicated can only in exceptions be economically justified.